Non-Invasive Delivery of dsGST Is Lethal to the Sweet Potato Whitefly, Bemisia tabaci (G.) (Hemiptera: Aleyrodidae)

Knockout of ecdysis triggering hormone receptor (ETHr) gene adversely affects the nymphal molting and adult reproduction in Bemisia tabaci

Bemisia tabaci (Gennadius) is one of the most dangerous polyphagous pests in the world causing damage to various crops by sucking sap during the nymphal and adult stages. Chemical management of whiteflies is challenging because of the emergence of pesticide resistance. RNA interference has been well established in whitefly to study the functions of various genes. G-protein coupled receptors (GPCRs) are important targets for development of new generation insecticides. In this study, Ecdysis triggering hormone receptor (ETHr) gene expression was recorded in different stages of whitefly and its function has been studied through RNAi. The expression of ETHr is highest in third-instar nymphs followed by other nymphal instars, pupae and newly emerged adults. Silencing of ETHr resulted in significantly higher adult mortality (68.88%), reduced fecundity (4.46 eggs /female), reduced longevity of male and female (1.05 and 1.40 days, respectively) when adults were fed with dsETHr @ 1.0 μg/μl. Silencing of ETHr in nymphs lead to significantly higher mortality (81.35%) as compared to control. This study confirms that ETHr gene is essential for growth and development of whitefly nymphs and adults. Hence, it can be future target for developing dsRNA based insecticides for management of whitefly.

Effectiveness of chitosan nanohydrogel mediated encapsulation of EcR dsRNA against the whitefly, Bemisia tabaci Asia-I (Gennedius) (Hemiptera: Aleyordidae)

Bemisia tabaci is a global invasive pest causing substantial loss on several economically important crops and has developed a very high level of resistance to insecticides making current management practices ineffective. Thus, the novel pest management strategy like RNA interference (RNAi) has emerged as a potential molecular tool in the management of insect pests particularly B. tabaci. The present study investigated RNAi mediated silencing of the Ecdysone Receptor (EcR) gene in B. tabaci Asia-I using biodegradable Chitosan Nanoparticles (CNPs) hydrogel containing EcR dsRNA. The formation of nanohydrogel and dsRNA loading were characterized by gel retardation assay, scanning electron microscopy (SEM); transmission electron microscopy (TEM) and Fourier transform infrared microscopy (FTIR). The stability of CNPs/dsRNA was assessed by exposure to direct sunlight and UV light for different time periods. The CNPs/dsRNA exhibited increased stability over the untreated control and further confirmed by bioassay studies which yielded mortality over 80% and effectively down regulated the expression of the EcR gene as confirmed by qRT-PCR analysis. These investigations provide potential avenues for advancing innovative pest management strategies using biopolymer CNPs hydrogel, which can enhance the efficiency of dsRNA as a safe and targeted solution in the management of whiteflies.

CYP6CX2 and CYP6CX3 mediate thiamethoxam resistance in field whitefly, Bemisia tabaci (Hemiptera:Aleyrodidae)

Cytochrome P450 monooxygenases (P450s) are well-known for their crucial roles in the detoxification of xenobiotics. However, whether CYP6CX2 and CYP6CX3, 2 genes from our Bemisia tabaci (B. tabaci) MED/Q genome data were associated with detoxification metabolism and confer resistance to thiamethoxam is unclear. In this study, we investigated the role of CYP6CX2 and CYP6CX3 in mediating whitefly thiamethoxam resistance. Our results showed that mRNA levels of CYP6CX2 and CYP6CX3 were up-regulated after exposure to thiamethoxam. Transcriptional levels of 2 genes were overexpressed in laboratory and field thiamethoxam resistant strains by RT-qPCR. These results indicate that the enhanced expression of CYP6CX2 and CYP6CX3 appears to confer thiamethoxam resistance in B. tabaci. Moreover, linear regression analysis showed that the expression levels of CYP6CX2 and CYP6CX3 were positively correlated with thiamethoxam resistance levels among populations. The susceptibility of whitefly adults was markedly increased after silencing 2 genes by RNA interference (RNAi) which further confirming their major role in thiamethoxam resistance. Our findings provide information to better understand the roles of P450s in resistance to neonicotinoids and suggest that these genes may be applied to develop target genes for sustainable management tactic of agricultural pests such as B. tabaci.

RNA meets toxicology: efficacy indicators from the experimental design of RNAi studies for insect pest management

RNA interference (RNAi) selectively targets genes and silences their expression in vivo, causing developmental defects, mortality and altered behavior. Consequently, RNAi has emerged as a promising research area for insect pest management. However, it is not yet a viable alternative over conventional pesticides despite several theoretical advantages in safety and specificity. As a first step toward a more standardized approach, a machine learning algorithm was used to identify factors that predict trial efficacy. Current research on RNAi for pest management is highly variable and relatively unstandardized. The applied random forest model was able to reliably predict mortality ranges based on bioassay parameters with 72.6% accuracy. Response time and target gene were the most important variables in the model, followed by applied dose, double-stranded RNA (dsRNA) construct size and target species, further supported by generalized linear mixed effect modeling. Our results identified informative trends, supporting the idea that basic principles of toxicology apply to RNAi bioassays and provide initial guidelines standardizing future research similar to studies of traditional insecticides. We advocate for training that integrates genetic, organismal, and toxicological approaches to accelerate the development of RNAi as an effective tool for pest management. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Effect of RNAi targeting CYP6CY3 on the growth, development and insecticide susceptibility of Aphis gossypii by using nanocarrier-based transdermal dsRNA delivery system

RNA interference (RNAi) has been proved to be a viable method for agricultural pest control. Due to the limited uptake of dsRNA in hemiptera insects, this study used nanocarrier SPc (star polycation) transdermal delivery systems to deliver two truncated fragments (P1/P2) dsRNA of the CYP6CY3 for silencing this target gene in Aphis gossypii. After the cotton aphid was sprayed with the SPc + dsP1/P2 mixture, the expression level of target gene in SPc + dsP1 treatment group was not different from that in dsP1 group at 24 h, 48 h, and significantly lower than that in dsP1 group at 60 h, 72 h, respectively; and the expression level of target gene in SPc + dsP2 treatment group was not different from that in dsP2 group at 24 h, and significantly lower than that in dsP2 group from 48 h, 60 h, 72 h, respectively. In addition, the expression level was continuously silenced after spraying the SPc + dsP1/P2 mixture and significant reduced by 79.7% and 84.3% at 48 h compared with the H₂O control group, the mortality rate reached 48.09% and 43.18% at 84 h, respectively. And the cumulative reproduction number of cotton aphids also decreased, but the cumulative death number of newborn nymphs had an increase trend, compared with the control groups. Bioassays after RNAi showed that the silencing of CYP6CY3 increased the susceptibility of the 4^th instar aphid to imidacloprid, and increased mortality by 67.21% and 58.69% at 96 h, respectively. The life table parameters of the offspring from the 4^th instar cotton aphids from the SPc + dsP1/P2 treatment groups showed that the offspring had a longer pre-reproductive period and post-reproductive period. The intrinsic growth rate was 0.231 ± 0.005, 0.210 ± 0.013 and the finite growth rate was 1.260 ± 0.007 and 1.234 ± 0.016 in the SPc + dsP1/P2 treatment group, these two parameters of the two groups were lower than that of the corresponding control，the population doubling time of the two groups was prolonged and the developmental duration was delayed. These results indicate that CYP6CY3 plays a key role in the growth, development, reproduction and detoxification ability in cotton aphids, and may be as a potential RNAi target for controlling aphids, laying the foundation for the development of new environmentally-friendly RNA pesticides in this field.

Tiny Flies: A Mighty Pest That Threatens Agricultural Productivity-A Case for Next-Generation Control Strategies of Whiteflies

Simple Summary

Despite being a pest of global importance, effective management of whiteflies by the implication of environmentally friendly approaches is still a far-reaching task. In this review, we have tried to bring the readers’ attention to next-generation control strategies such as RNA interference and genetic modifications of plants for the expression of anti-whitefly proteins. These strategies offer huge promise to provide an effective and sustainable solution to the problem of whiteflies, either in isolation or in combination with other widely used practices under the regimes of integrated pest management. Focus has also been given to advanced technologies such as nanotechnology and genome editing, with promising prospects for field applications. The importance, applicability, and demand of these technologies for the control of whiteflies have been highlighted. We have also attempted to present the holistic picture of challenges in the path of commercial application of these promising technologies. To underline the pest status of whiteflies concisely, we have enlisted all economically important species of the pest along with their host plants/crops across the world. A comprehensive list of various insecticides of chemical, microbial, and botanical origin, applied in the field for the control of sweetpotato whitefly along with their resistance status, ecotoxicities, and effects on biological control agents, has been provided for readers.

Abstract

Whiteflies are a group of universally occurring insects that are considered to be a serious pest in their own way for causing both direct and indirect damages to crops. A few of them serve as vectors of plant viruses that are detrimental to the crop in question and cause an actual loss in productivity. A lot of attention is focused on pest control measures under the umbrella of IPM. In this review, we attempt to summarize the existing literature on how and why whiteflies are a serious concern for agriculture and society. We reviewed why there could be a need for fresh insight into the ways and means with which the pest can be combated. Here, we have emphasized next-generation strategies based on macromolecules, i.e., RNA interference and genetic engineering (for the expression of anti-whitefly proteins), as these strategies possess the greatest scope for research and improvement in the future. Recent scientific efforts based on nanotechnology and genome editing, which seem to offer great potential for whitefly/crop pest control, have been discussed. Comprehensive apprehensions related to obstacles in the path of taking lab-ready technologies into the farmers’ field have also been highlighted. Although the use of RNAi, GM crops, nanotechnologies, for the control of whiteflies needs to be evaluated in the field, there is an emerging range of possible applications with promising prospects for the control of these tiny flies that are mighty pests.

Downregulation of dystrophin expression in pupae of the whitefly Bemisia tabaci inhibits the emergence of adults

The whitefly Bemisia tabaci is a major pest to agriculture. Adults are able to fly for long distances and to colonize staple crops, herbs and ornamentals, and to vector viruses belonging to several important taxonomic groups. During their early development, whiteflies mature from eggs through several nymphal stages (instars I to IV) until adults emerge from pupae. We aim at reducing whitefly populations by inhibiting the emergence of adults from nymphs. Here we targeted dystrophin, a conserved protein essential for the development of the muscle system in humans, other animals and insects. We have exploited the fact that whitefly nymphs developing on tomato leaves feed from the plant phloem via their stylets. Thus, we delivered dystrophin-silencing double-stranded RNA to nymphs developing on leaves of tomato plantlets with their roots bathing in the silencing solution. Downregulation of dystrophin expression occurred mainly in pupae. Dystrophin silencing induced also the downregulation of the dystrophin-associated protein genes actin and tropomyosin, and disrupted F-actin. Most significantly, the treatment inhibited the emergence of adults from pupae, suggesting that targeting dystrophin may help to restrain whitefly populations. This study demonstrates for the first time the important role of dystrophin in the development of a major insect pest to agriculture.

Potential of RNA interference in the study and management of the whitefly, Bemisia tabaci

Whiteflies cause considerable losses to crops, directly by feeding, and indirectly by transmission of viruses. The current control methods consist of a combination of different control tactics, mainly still relying on unsafe and non-ecofriendly chemical control. RNA interference (RNAi) is a post-transcriptional gene-silencing strategy in which double-stranded RNA (dsRNA), corresponding specifically to a target gene, is introduced in a target organism. Research on RNAi in the previous decade has shown its success as a potential insect control strategy, which can be highly species-specific and environment friendly. In whiteflies, the success of dsRNA delivery through the oral route opened possibilities for its management through plant-mediated RNAi. To date, several genes have been targeted in whiteflies through RNAi and these assays demonstrated its potential to manage whiteflies at lab level. However, further research and investments are needed to move toward an application at field level. In this review, for the first time, we collected the literature on genes targeted for silencing via RNAi in whiteflies and discuss the potential of RNAi in whitefly pest control. We also discuss likely delivery methods, including transgenic in planta delivery and symbiont-mediated delivery, and its potential for studying and interfering with insecticide resistance mechanisms and virus transmission by whiteflies.

Targeting detoxification genes by phloem-mediated RNAi: A new approach for controlling phloem-feeding insect pests

Many phloem-feeding insects are considered severe pests of agriculture and are controlled mainly by chemical insecticides. Continued extensive use of these inputs is environmentally undesirable, and also leads to the development of insecticide resistance. Here, we used a plant-mediated RNA interference (RNAi) approach, to develop a new control strategy for phloem-feeding insects. The approach aims to silence "key" detoxification genes, involved in the insect's ability to neutralize defensive and toxic plant chemistry. We targeted a glutathione S-transferase (GST) gene, BtGSTs5, in the phloem-feeding whitefly Bemisia tabaci, a devastating global agricultural pest. We report three major findings. First, significant down regulation of the BtGSTs5 gene was obtained in the gut of B. tabaci when the insects were fed on Arabidopsis thaliana transgenic plants expressing dsRNA against BtGSTs5 under a phloem-specific promoter. This brings evidence that phloem-feeding insects can be efficiently targeted by plant-mediated RNAi. Second, in-silico and in-vitro analyses indicated that the BtGSTs5 enzyme can accept as substrates, hydrolyzed aliphatic- and indolic-glucosinolates, and produce their corresponding detoxified conjugates. Third, performance assays suggested that the BtGSTs5 gene silencing prolongs the developmental period of B. tabaci nymphs. Taken together, these findings suggest that BtGSTs5 is likely to play an important role in enabling B. tabaci to successfully feed on glucosinolate-producing plants. Targeting the gene by RNAi in Brassicaceae cropping systems, will likely not eliminate the pest populations from the fields but will significantly reduce their success over the growing season, support prominent activity of natural enemies, eventually allowing the establishment of stable and sustainable agroecosystem.

RNAi-mediated resistance to whitefly (Bemisia tabaci) in genetically engineered lettuce (Lactuca sativa)

RNA interference (RNAi)-based transgenic technologies have evolved as potent biochemical tools for silencing specific genes of plant pathogens and pests. The approach has been demonstrated to be useful in silencing genes in insect species. Here, we report on the successful construction of RNAi-based plasmid containing an interfering cassette designed to generate dsRNAs that target a novel v-ATPase transcript in whitefly (Bemisia tabaci), an important agricultural pest in tropical and sub-tropical regions. The presence of the transgene was confirmed in T0 and T1 generations of transgenic lettuce lines, segregating in a Mendelian fashion. Seven lines were infested with whiteflies and monitored over a period of 32 days. Analysis of mortality showed that within five days of feeding, insects on transgenic plants showed a mortality rate of 83.8-98.1%. In addition, a reduced number of eggs (95 fold less) was observed in flies feeding on transgenic lettuce plants than insects on control lines. Quantitative reverse transcription PCR showed decreased expression level of endogenous v-ATPase gene in whiteflies feeding on transgenic plants. This technology is a foundation for the production of whitefly-resistant commercial crops, improving agricultural sustainability and food security, reducing the use of more environmentally aggressive methods of pest control.

Silencing of Target Chitinase Genes via Oral Delivery of dsRNA Caused Lethal Phenotypic Effects in Mythimna separata (Lepidoptera: Noctuidae)

Mythimna separata walker (Lepidoptera: Noctuidae) is a polyphagous, migratory corn pest. Outbreak of M. separata has led to severe damage to corn production recently in China. RNAi (RNA interference) is a gene silencing technology applied both in model and non-model organisms, and it is especially useful for the latter in which the reverse genetic research tools are not available. RNAi approach was broadly investigated in many plant pathogens and was used for the generation of anti-pest transgenic plants. We are proposing to use this technology to silence M. separata endogenous genes, thereby, providing a biocontrol method for this insect. Feeding of dsRNAs for target Chitinase genes resulted in substantial decreases of their transcript levels in M. separata. Furthermore, silencing of target Chitinase genes led to phenotypic effects such as reduced body weight and increased mortality. Our study provided both reverse genetic research tool and potential control strategy for this insect species.

The Whitefly Bemisia tabaci Knottin-1 Gene Is Implicated in Regulating the Quantity of Tomato Yellow Leaf Curl Virus Ingested and Transmitted by the Insect

The whitefly Bemisia tabaci is a major pest to agricultural crops. It transmits begomoviruses, such as Tomato yellow leaf curl virus (TYLCV), in a circular, persistent fashion. Transcriptome analyses revealed that B. tabaci knottin genes were responsive to various stresses. Upon ingestion of tomato begomoviruses, two of the four knottin genes were upregulated, knot-1 (with the highest expression) and knot-3. In this study, we examined the involvement of B. tabaci knottin genes in relation to TYLCV circulative transmission. Knottins were silenced by feeding whiteflies with knottin dsRNA via detached tomato leaves. Large amounts of knot-1 transcripts were present in the abdomen of whiteflies, an obligatory transit site of begomoviruses in their circulative transmission pathway; knot-1 silencing significantly depleted the abdomen from knot-1 transcripts. Knot-1 silencing led to an increase in the amounts of TYLCV ingested by the insects and transmitted to tomato test plants by several orders of magnitude. This effect was not observed following knot-3 silencing. Hence, knot-1 plays a role in restricting the quantity of virions an insect may acquire and transmit. We suggest that knot-1 protects B. tabaci against deleterious effects caused by TYLCV by limiting the amount of virus associated with the whitefly vector.

The autophagy pathway participates in resistance to tomato yellow leaf curl virus infection in whiteflies

Macroautophagy/autophagy plays an important role against pathogen infection in mammals and plants. However, little has been known about the role of autophagy in the interactions of insect vectors with the plant viruses, which they transmit. Begomoviruses are a group of single-stranded DNA viruses and are exclusively transmitted by the whitefly Bemisia tabaci in a circulative manner. In this study, we found that the infection of a begomovirus, tomato yellow leaf curl virus (TYLCV) could activate the autophagy pathway in the Middle East Asia Minor 1 (MEAM1) species of the B. tabaci complex as evidenced by the formation of autophagosomes and ATG8-II. Interestingly, the activation of autophagy led to the subsequent degradation of TYLCV coat protein (CP) and genomic DNA. While feeding the whitefly with 2 autophagy inhibitors (3-methyladenine and bafilomycin A1) and silencing the expression of Atg3 and Atg9 increased the viral load; autophagy activation via feeding of rapamycin notably decreased the amount of viral CP and DNA in the whitefly. Furthermore, we found that activation of whitefly autophagy could inhibit the efficiency of virus transmission; whereas inhibiting autophagy facilitated virus transmission. Taken together, these results indicate that TYLCV infection can activate the whitefly autophagy pathway, which leads to the subsequent degradation of virus. Furthermore, our report proves that an insect vector uses autophagy as an intrinsic antiviral program to repress the infection of a circulative-transmitted plant virus. Our data also demonstrate that TYLCV may replicate and trigger complex interactions with the insect vector.

Diet-Delivered dsRNAs for Juvenile Hormone-Binding Protein and Vacuolar ATPase-H Implied Their Potential in the Management of the Melon Aphid (Hemiptera: Aphididae)

RNA interference is a sequence-specific gene silencing mechanism mediated by double-stranded RNA (dsRNA), which has been harnessed as a useful tool in devising novel insect pest management strategies for various pests such as melon aphid, Aphis gossypii (Glover). In the current study, we cloned and sequenced juvenile hormone-binding protein (JHBP) and vacuolar ATPase subunit H (V-ATPase-H) from A. gossypii. We also showed the effectiveness of diet-mediated delivery of dsRNA for JHBP and V-ATPase-H, which silenced the above genes and resulted in mortality. The extent of silencing and mortality were similar for both genes up until 96 h. Bioassay results revealed that the target genes were silenced variably, 1.0 µg/µl concentration having a more profound effect than 0.5 and 0.25 µg/µl concentration in reducing the cognate mRNA transcript level. Results indicated a 9.56–73.21% down regulation (across time and concentrations for both the genes) that resulted in the mortality of A. gossypii. Mortality was in the range of 10–63% for both these genes. Thus, the current study demonstrated the potentiality of both JHBP and V-ATPase-H as excellent targets for the management of A. gossypii.

Small RNA Regulators of Plant-Hemipteran Interactions: Micromanagers with Versatile Roles

Non-coding small RNAs (sRNAs) in plants have important roles in regulating biological processes, including development, reproduction, and stress responses. Recent research indicates significant roles for sRNA-mediated gene silencing during plant-hemipteran interactions that involve all three of these biological processes. Plant responses to hemipteran feeding are determined by changes in the host transcriptome that appear to be fine-tuned by sRNAs. The role of sRNA in plant defense responses is complex. Different forms of sRNAs, with specific modes of action, regulate changes in the host transcriptome primarily through post-transcriptional gene silencing and occasionally through translational repression. Plant genetic resistance against hemipterans provides a model to explore the regulatory roles of sRNAs in plant defense. Aphid-induced sRNA expression in resistance genotypes delivers a new paradigm in understanding the regulation of R gene-mediated resistance in host plants. Unique sRNA profiles, including changes in sRNA biogenesis and expression can also provide insights into susceptibility to insect herbivores. Activation of phytohormone-mediated defense responses against insect herbivory is another hallmark of this interaction, and recent studies have shown that regulation of phytohormone signaling is under the control of sRNAs. Hemipterans feeding on resistant plants also show changes in insect sRNA profiles, possibly influencing insect development and reproduction. Changes in insect traits such as fecundity, host range, and resistance to insecticides are impacted by sRNAs and can directly contribute to the success of certain insect biotypes. In addition to causing direct damage to the host plant, hemipteran insects are often vectors of viral pathogens. Insect anti-viral RNAi machinery is activated to limit virus accumulation, suggesting a role in insect immunity. Virus-derived long sRNAs strongly resemble insect piRNAs, leading to the speculation that the piRNA pathway is induced in response to viral infection. Evidence for robust insect RNAi machinery in several hemipteran species is of immense interest and is being actively pursued as a possible tool for insect control. RNAi-induced gene silencing following uptake of exogenous dsRNA was successfully demonstrated in several hemipterans and the presence of sid-1 like genes support the concept of a systemic response in some species.